In a turbomachine, stages of variable-pitch stator vanes are mounted between rotor disks of a compressor. These variable-pitch vanes are supported by a casing of the stator and can be adjusted in position about their axes to optimize the flow of the gases in the engine of the turbomachine.
In the current art, a variable-pitch vane comprises a substantially cylindrical axial pivot at each of its ends, these pivots defining the axis of rotation of the vane. The radially external pivot, termed actuating pivot, is fitted into the housing of a substantially radial or slightly inclined duct of a casing of the turbomachine and is connected by a lever to an actuating ring operated by a ram, an electric motor, or the like. The rotation of the actuating ring is transmitted by the levers to the external pivots of the vanes and causes them to turn about their axes.
The radially internal pivot, termed guide pivot, is fitted into a substantially radial or slightly inclined housing of an internal ring mounted on an annular shroud which bears blocks of abradable material. These blocks of abradable material are intended to cooperate with annular seal teeth provided on the rotor of the turbomachine in order to limit the recirculation of air, from upstream to downstream and from downstream to upstream, radially inside the ring.
The operating cycle of the turbomachine comprises transient modes leading to relative movement in the axial and radial directions between the rotor and the stator, which results in considerable wear on the blocks of abradable material caused by the seal teeth of the rotor rubbing against these blocks.
During the takeoff phase for example, the rotor moves in the radial direction toward the stator and the seal teeth deeply penetrate the blocks of abradable material, forming annular channels in these blocks. The relative movements between the rotor and the stator lead to an axial widening of these channels. During cruise mode, the seal teeth are situated radially set back from these blocks and facing previously formed channels, this resulting in considerable radial clearances between the tips of the seal teeth and the blocks of abradable material, and hence in a low degree of efficiency of the labyrinth seal formed by the seal teeth of the rotor.
Moreover, the relative movements in the axial direction between the rotor and the stator are difficult to predict. To prevent the rotor and the stator from coming into contact with one another during the aforementioned transient modes, the axial clearances between the rotor and the stator are generally increased. However, this solution is not satisfactory since it entails increasing the axial dimension of the engine and the mass of the turbomachine.